This Phase II study is a multi-center, randomized, placebo-controlled trial designed to assess the safety, pharmacokinetics, pharmacodynamics, immunogenicity, and biologic activity of BMS-188667 (CTLA4Ig) in patients with psoriasis vulgaris. Eligable patients will have stable plaque psoriasis of greater than six months duration, and involvement of at least 10% body surface area (BSA), and will have failed at least one standard anti-psoriatic therapy for toxicity and/or inefficacy. Information from this study will be used in the development of dosing schedules in Phase III studies in this patient population.